The present invention is directed to a method and system for employing customer premises equipment (CPE) as an agent in communication network maintenance; and, in a particular embodiment, for employing CPE to aid in measuring the effect of network loss on received sound quality so that such loss may be compensated and sound quality thereby improved.
TrueVoice(copyright) is a telephone network sound enhancement technology invented by ATandT Corp. TrueVoice provides ATandT customers with xe2x80x9ccloser,xe2x80x9d clearer sounding voice communications on telephone calls carried by the ATandT network. TrueVoice applies both a frequency independent gain (a constant gain across all frequencies in the telephone bandwidth) and a frequency selective gain (sometimes referred to as xe2x80x9cpre-emphasisxe2x80x9d) to telephone connections. Technical features of TrueVoice are described in U.S. Pat. Nos. 5,195,132; 5,333,195; and 5,206,902 (which are hereby incorporated by reference).
FIG. 1 is a diagram of a typical telephone network connection employing TrueVoice. It includes CPE 20 and 21 at two end points for a calling party and a called party, respectively, and analog and digital network components in between. CPE 20, 21 is, e.g., a conventional telephone. CPE 20, 21 is connected to a conventional hybrid converter 22, 23 at telephone central offices via local loops 31, 34. Each hybrid 22, 23 converts bidirectional signal transmission in a two-wire telephone line to two unidirectional signal paths (two wires each). The calling party""s xe2x80x9ctransmit pathxe2x80x9d includes local loop 31, network paths 35 and 36 and local loop 34. This transmit path is also the called party""s xe2x80x9creceive path.xe2x80x9d The called party""s transmit path includes local loop 34, network paths 38 and 37, and local loop 31. This transmit path is also the calling party""s receive path. (Local loops 31 and 34 are common to both calling and called parties"" transmit and receive paths.) The point of the network between paths 35 and 36/37 and 38 is said to be at 0 xe2x80x9cTLPxe2x80x9d (or transmission level point). This point may be conveniently used as a reference for gain or loss experienced at different points in the network.
Signals on the calling party transmit network path 35 are processed by a D/A-A/D converter 24, which is conventional equipment located at the calling party""s local central office. (For clarity of presentation of the invention, conventional switches associated with the local central offices are not shown.) A long-distance telephone network switch, e.g., a No. 4 Electronic Switching System 26 (4ESS) in the ATandT Network, is connected to the D/A-A/D 24 converter of the local central office. The 4ESS 26 is then connected to a special automatic volume control filter (AVC) 30 which, as shown in FIG. 1, includes, for example, TrueVoice(copyright) elements 32 and 33. As a matter of general background, automatic volume control filters are conventional, for example, those described in U.S. Pat. Nos. 4,499,578 and 4,535,445, which are hereby incorporated by reference. TrueVoice element 33 applies the sound enhancement for speech signals spoken at CPE 20 for transmission to CPE 21. Element 32 applies echo cancellation to diminish an echo of speech signals (originally spoken at CPE 21) returning to CPE 21. As shown in FIG. 1, similar connections are used in network path 36 to deliver speech from the calling party. This path includes a xe2x88x926 dB attenuator 29, intentionally inserted into the network, typically by the called party""s Local Exchange Carrier (LEC), to further mitigate echo in a long distance connection (it is not needed in a local connection). The Figure further illustrates similar connections for network path 38 (which is like network path 35) and network path 37 (which is like network path 36). (Although much of the discussion which follows is presented from the point of view of the calling party""s transmit path (which is the same as the called party""s receive path), such discussion has applicability to the called party""s transmit path/calling party""s receive path, with for example, the roles of elements 32 and 33 reversed.)
The part of the network which is digitalxe2x80x94that part between and including D/A-A/D converters 24, 25xe2x80x94exhibits no unintentional loss (there are xe2x88x926 dB attenuators 28, 29, however, which are intentionally placed in the circuit). The analog part of the networkxe2x80x94the balance of the network diagram of FIG. 1xe2x80x94does suffer unintentional loss, however. This loss is variable depending on the length of the local loop 31, 34 between the CPE 20, 21 and the central office. In addition, the level of a speech signal presented to the analog part of the network is variable, depending on the CPE (telephone) 20, 21 microphone efficiency, as well as how loudly a person is speaking into the microphone and how close the person""s mouth is to the microphone. As shown in the Figure, the average loss on the analog portion of the calling party""s transmit pathxe2x80x94referred to as TOLR (telephone+local loop loss) is xe2x88x9246 dB.
As discussed in greater detail in the referenced patents, TrueVoice(copyright) 33 sound enhancement operates to mitigate the effect of signal loss in a telephone network connection for signals traveling from the calling party to the called party. TrueVoice 33 sits in the middle of the digital network and adds gain of a fixed amount (4 dB) to a computed input power of a transmitted speech signal. FIG. 2 illustrates this. The power of the transmitted (input) signal is computed over a time interval. For example, the signal may have an average power over the interval of xe2x88x9221 dB. TrueVoice will amplify the signal such that the signal will have an average power of 4 dB better (or xe2x88x9217 dB). If the average power of the input signal is xe2x88x9217 dB, the output power will be raised to xe2x88x9214 dB. Through its combination of pre-emphasis (base boost) and the AVC, TrueVoice 33 compensates some or all of the network path 31, 35 attenuation, as well as CPE 20 efficiency variation, to improve how speech carried over the telephone connection xe2x80x9csoundsxe2x80x9d to someone listening.
Although there is an optimal TrueVoice 33 output power level to which the network signal could be adjusted, TrueVoice employs a conservative boost of, e.g., a constant 4 dB, to compensate for attenuation suffered in the paths 31 and 35 of the network. Unfortunately, there are several variables related to the paths 34 and 36 of the network which affect the amount of signal loss a speech signal may suffer in transmission over a telephone circuit. For example, the attenuator 29 is not always present in a long distance connection. Network response variability is also caused by variation in local-loop 34 length carrying received signals and variability in the efficiency of CPE 21""s electric-to-acoustic transduction. This response variability can cause, among other things, variability of objective loudness as perceived by telephone customers. Moreover, because TrueVoice(copyright) 33 applies a conservative gain mapping (4 dB, for example) when administering active volume control, called parties connected on long loops or loops that cause great attenuation may not be able to perceive all the benefits of TrueVoice(copyright) 33. Since TrueVoice 33 does not know what the loss will be on the paths 36, 34 of the network, it does not compensate for such loss and, in fact, provides a relatively conservative maximum gain because of this.
The present invention is directed to the use of CPE as an agent of the network to assist in providing network maintenance. An illustrative embodiment of the invention is directed to improving network sound enhancement systems, such as TrueVoice. In this embodiment, the TrueVoice network element 41 (see FIG. 3) emits a first signal at the beginning of an ordinary telephone call (where the reference numeral 41 has been used, rather than numeral 33, to indicate a TrueVoice element employing features related to an embodiment of the invention). This first signal is illustratively a sub-audible 25 Hz tone referred to as a xe2x80x9ctagxe2x80x9d signal (which the network otherwise uses with conventional TrueVoice to indicate to other network components that TrueVoice is being applied; see U.S. Pat. No. 5,206,902). When the CPE senses the first signal, it measures the loss in the signal (knowing a priori at what level the signal was transmitted) and responds with a signal of its ownxe2x80x94a calibration signal. This calibration signal represents a measure of the first signal loss sensed by the CPE on the call. Illustratively, the calibration signal has the loss value encoded therein. The TrueVoice network 41 element decodes the measured loss information and adjusts the transmitted telephone signal (which is sent to the CPE 42) to account for the measured loss. In this embodiment, the system assumes that the loss measured at 25 Hz is applicable at other audible frequencies. Because signal loss is measured, network sound enhancement can be tailored on a call-by-call basis.
Embodiments of the present invention may also employ a wide-band signal (in addition to or instead of the tag signal) to allow measurement of loss at various frequencies in the audible range. The coded calibration signal then represents loss at these various frequencies. TrueVoice network element 41 adjusts the transmitted signal level differently at different frequencies so as to obtain a more frequency-dependent enhancement to network sound. The calibration signal may be of any suitable type and may include DTMF signals, coded sub-audible signals, spread spectrum signals, data signals from a voice-over-data modem signals, etc.
A further alternative embodiment is one in which the CPE does not measure the first signal, but merely provides a xe2x80x9cloop backxe2x80x9d of such signal to the network. The loop back signal level is then measured by the TrueVoice network element and loss is then estimated. In this embodiment, the loss measured by the network includes loss suffered in both a receive and transmit path (during both the transmission of the first signal, as well as the loop back transmission from the CPE). Therefore, loss in just the receive path must be estimated as, for example half of the total loss measured by the network element.
In these embodiments, the CPE is participating in the maintenance of the telephone networkxe2x80x94in this case, the maintenance of network sound quality. This maintenance behavior shifts the paradigm of the xe2x80x9cintelligent network but xe2x80x98dumbxe2x80x99 terminalxe2x80x9d (CPE) to one which employs xe2x80x9csmartxe2x80x9d CPE providing assistance to the network in performing network maintenance.